The preparation of isocyanates by phosphenation of the corresponding amines can in principle be carried out by means of a liquid-phase phosgenation or a gas-phase phosgenation. Unlike the gas-phase phosgenation, the reaction in the liquid-phase phosgenation is carried out at low temperatures, and vaporization of the starting materials is not necessary.
In liquid-phase phosgenation, an amine-comprising feed stream in liquid form is fed in. This is mixed with a phosgene-comprising feed stream. The phosgene can here be dissolved in an inert solvent. The phosgene-comprising feed stream is subsequently injected into a mixing device in which it mixes with the amine-comprising feed stream. The amine and the phosgene react with liberation of HCl to form the corresponding isocyanate.
Rapid mixing of the amine with the phosgene is necessary since, at an insufficient phosgene concentration, the isocyanate formed reacts with the excess amine to form urea or other troublesome, high-viscosity and solid by-products. For this reason, rapid mixing and a short residence time in the reaction chamber are necessary.
A process for the liquid-phase phosgenation of amines for preparing isocyanates is described, for example, in WO 2010/015667 A1.
In gas-phase phosgenation, an amine-comprising feed stream and a phosgene-comprising feed stream, each in the gaseous state, are mixed. The amine and the phosgene react with liberation of HCl to form the corresponding isocyanate. The amine-comprising feed stream is generally present in liquid form and has to be vaporized and optionally superheated before mixing with the phosgene-comprising stream.
Owing to the low vapor pressure in particular of the diamines, the vaporization is carried out at elevated temperature. However, this can cause decomposition reactions of the amines or diamines, for example deaminations, demethylations and dimerizations, which have an adverse effect on the selectivity of the overall process.
In addition, reactions quickly commence on contacting of the two feed streams as a result of the high, temperatures. Apart from phosgenation of the amine to form isocyanate, it is possible for undesirable secondary and subsequent reactions to take place. Thus, for example, isocyanate which has already been formed can react with as yet unreacted amine to form a urea. Furthermore, carbodiimides and cyanurates can also be formed. This firstly affects the selectivity of the process, and, secondly, solid by-products which have been formed can lead to blockages and thus have an adverse effect on the running time of the plant. Efforts are therefore generally made to mix the feed streams as quickly as possible in order to avoid, as far as possible, mixing ratios which accelerate the formation of secondary components.
A process for preparing (poly)isocyanates in the gas phase with optimized mixing of the reactants is described, for example, in EP 1 319 655 A2.
Thus, mixing of the starting materials and the residence time of the reaction mixture in the corresponding reaction spaces are critical parameters both in gas-phase phosgenation and in liquid-phase phosgenation. The plants for preparing isocyanates by phosgenation of amines therefore have to be matched to the specific requirements in respect of rapid mixing of the feed streams and a narrow residence time window. Plants for the phosgenation of amines are designed essentially for the maximum streams of materials or for the respective nominal load. This means both mixing devices such as nozzles and also the reaction spaces, for example residence reactors, operate at the nominal load in the optimal region with optimized yield, purity of the products, etc. However, if the plant is not operating at full load, i.e. it is operated at only part of the nominal load, the residence times, for example; alter and the plant is no longer operating in the optimal region. This is the case, for example, during start-up and running-down, part loading of the plant or malfunctions in the plant. In these cases of reduced load, both the mixing devices and the residence reactors do not operate in the optimal region. The consequences are decreases in yield, fouling problems and/or reductions in quality.
It was therefore an object of the present invention to provide a process for preparing isocyanates by reading the corresponding amines with phosgene, which process can also be carried out at various load states without the above-described problems; in particular, mixing and/or the reaction should occur in the respective optimized residence time window even when the plant is operated at part load.